1. Field of the Invention
The present invention relates to gain control circuits, and more particularly, a novel and improved temperature compensated gain control circuit including a field effect transistor.
2. Description of the Prior Art
FIG. 1 is a block diagram of a radio frequency transceiver used for mobile communications (such as in cellular telephony) as in U.S. Pat. No. 5,107,225. A receiver portion (receiver) 1 includes a low-noise amplifier 2, a first frequency converter (mixer) 3, a three-stage AGC (automatic gain control) amplifier having stages 4-a, 4-b, 4-c, a second frequency converter (mixer) 5, an intermediate frequency amplifier 6, and a base band signal processing circuit (not shown) for reception which is connected to the output terminal of the intermediate frequency amplifier 6. Amplifier 6 includes amplifier 6-a, first bandpass filter 6-b, second bandpass filter 6-c, and detector amplifier 6-d. The frequency sources 3', 11' provide the local oscillators for the mixing process. First frequency converter 3 mixes a received signal (e.g. approximately 881 MHz) and an output of first local oscillator 3' (e.g. approximately 951 MHz) and outputs a first intermediate frequency signal (e.g. approximately 70 MHz) by frequency conversion. Second frequency converter 5 mixes the first intermediate frequency signal and an output of local oscillator 5' (e.g. approximately 75 MHz) and outputs a second intermediate frequency signal (e.g. approximately 5 MHz) by frequency conversion. The intermediate frequency amplifier 6 includes a detection function and thus outputs a D.C. voltage (a detection signal) indicating the post AGC signal level. The detection signal is used for AGC.
Differential amplifier 7 compares the detection signal to signal V.sub.REF. Signal V.sub.REF is a signal from the base band signal processing circuit, and is used for adjustment of the gain of the AGC amplifier stages 4-a, 4-b, 4-c. The output signal RX-V.sub.AGC of the differential amplifier 7 is input to each of the AGC amplifier stages 4-a, 4--b, 4--c to control the gain thereof. The AGC amplifier stages 4-a, 4-b, 4--c, the intermediate frequency amplifier 6, and the differential amplifier 7 form an AGC loop. The AGC amplifier stages 4-a, 4-b, 4-c, the intermediate frequency amplifier 6, the base band signal processing circuit, and the differential amplifier 7 form another control loop.
A transmitter portion (transmitter) 8 includes a base band signal processing circuit (not shown) for transmission, a first frequency converter 9 for converting the carrier frequency modulated by the base band signal (e.g., carrier frequency is approximately 5 MHZ) to the first intermediate frequency signal (e.g. approximately 70 MHz), a three-stage AGC amplifier 10-a, 10-b, 10-c, a second frequency converter 11, and a power amplifier 12. The first frequency converter 9 receives the output of the first local oscillator 9' (e.g. approximately 75 MHz) and outputs the first intermediate frequency signal. The second frequency converter 11 mixes the first intermediate frequency signal and the output of the second local oscillator 11' (e.g. approximately 906 MHz) and outputs the radio frequency signal (e.g. approximately 836 MHz). Adding circuit 13 adds AGC signal RX-V.sub.AGC of the receiver portion 1 to a transmission level setting signal TX-V.sub.GAIN CONT and outputs an AGC voltage TX-V.sub.AGC for control of the gain of the AGC transmitter amplifier stages 10-a, 10-b, 10-c. A signal transmitted from the base station to the mobile station includes power control information for controlling the transmission power from the mobile station to the base station. Receiver portion 1 of the mobile station receives the signal transmitted from the base station, and an internal circuit (not shown) of the mobile station varies the level of signal TX-V.sub.GAIN CONT based on the power control information. Signal TX-V.sub.GAIN CONT controls the transmission power by means of transmitter portion 8.
Duplexer 14 connects the transmit signal path and the receive signal path to the antenna. Antenna 15 is thereby conventionally used for both transmission and reception.
In the system of FIG. 1 transmission power is controlled in response to the level of the received signal. That is, when the level of the received signal is low, transmission power is increased, and when the received signal level is high, transmission power is reduced.
FIG. 2 shows detail of one stage of the AGC amplifier stages 4-a, 4-b, 4-c of the receiver portion 1 and one stage of the AGC amplifier stages 10-a, 10-b, 10-c of the transmitter portion 8. Each of the AGC amplifiers includes a dual gate MOSFET (metal oxide semiconductor field effect transistor) respectively Q4, Q1. An AGC voltage is applied to a second gate G2 of MOSFETs Q1, Q4. Power is supplied at the terminals labelled "+B".
In the above-described circuit, when the level of a received signal is high, the level of the detection signal increases, lowering AGC voltage RX-V.sub.AGC of the receiver portion 1 which is the output of the differential amplifier 7. Consequently, the voltage at the second gate G2 of each of the MOSFETs of the AGC amplifier stages 4-a, 4-b, 4--C is lowered, thus reducing the gain of the AGC amplifier stages 4-a, 4-b, 4-c. AGC voltage RX-V.sub.AGC of the receiver portion 1 is also applied to the second gate G2 of each of the MOSFETs of the AGC amplifier stages 10-a, 10-b, 10-c of the transmitter portion 8 via the adding circuit 13, thus reducing the gain of the AGC amplifier stages 10-a, 10-b, 10-c.
In a dual gate MOSFET amplifier where gain is controlled by gate voltage, the gain (vertical axis) changes with respect to gate voltage (horizontal axis) with various ambient temperatures as shown by FIG. 3a. At a high gate voltage, the gain at high ambient temperature (curve B) is lower than that at low ambient temperature (curve A). At a low gate voltage, the gain at high ambient temperature is higher than the gain at low ambient temperature.
Therefore when ambient temperature changes, amplifier gain may change even if the level of the received signal remains the same, thus changing the AGC voltage. The AGC voltage of the receiver portion 1 is applied to both the AGC amplifier of the receiver portion and the AGC amplifier stages 10-a, 10-b, 10-c of the transmitter portion 8. Even if the level of the received signal does not change, changes in the ambient temperature change the transmitter power of the transmission portion 8. Consequently, the desired relationship between received signal level and transmitter power may not occur, and hence the system may not operate as desired.